The IEEE 802.11 set of protocols includes 802.11(b) and 802.11(a). Also known as 802.11 High Rate or Wi-Fi (i.e., “wireless fidelity”), the 802.11(b) approach was approved by the IEEE in 1999 and is currently the mainstream technology adopted by wireless device manufacturers. Essentially using a Direct-Sequence Spread Spectrum (DSSS) technique, 802.11(b) uses a modulation scheme known as Complementary Code Keying (CCK) to transmit data signals at 11 megabits per second (Mbps) over an unlicensed portion of the radio frequency spectrum at around 2.4 GHz. IEEE 802.11(b) enabled a new generation of products to communicate wirelessly with an Ethernet-like connection. Unfortunately, however, the speed of 802.11(b) is only one-tenth that of its wired counterpart, IEEE 802.3.
The IEEE 802.11(a) standard was approved concurrently with 802.11(b), but utilizes Orthogonal Frequency Division Multiplexing (OFDM) as the modulation technique for signal transmission. OFDM is not compatible with 802.11(b) devices because they use CCK modulation. IEEE 802.11(a) technology can transmit data signals at up to 54 Mbps and operates in the 5 GHz frequency spectrum.
It would be desirable to extend the benefits of higher bit rate OFDM transmission to the 2.4 GHz band, which, between the two modulations, is the exclusive domain of the CCK scheme of 802.11(b). The IEEE 802.11(g) standard attempts to merge these operational characteristics together. IEEE 802.11(g) OFDM transmissions, however, are hidden from the legacy 802.11(b) nodes, because the 802.11(b) “physical carrier sense mechanism,” explained shortly, does not detect the OFDM carrier.
In the prior art, 802.11(g) nodes can fall back to the “virtual carrier sense mechanism” to protect OFDM transmissions from colliding (i.e., experiencing collisions) with transmissions using other modulations. The 802.11 medium access control (MAC) is based around a collision avoidance mechanism, meaning that nodes defer to an active transmission because they see that the shared channel (or “medium”) is busy. Their clear channel assessment is a mechanism that senses a physical carrier on the medium.
Furthermore, the MAC protocol defines a virtual carrier sense mechanism, in addition to the traditional physical carrier sense mechanism. To implement the virtual carrier sense mechanism, each node maintains a network allocation vector (NAV) counter that indicates whether the medium must be considered busy or not. After each frame reception at a node (whether the frame has been directed to the node or not), the node initializes its NAV counter with a duration value that is obtained from the duration field in the frame header of the received frame. Over time, this duration value decrements down until it reaches zero, indicating that it is presumptively safe to transmit. Conversely, a non-zero NAV value indicates that the virtual carrier sense (and the share channel) is busy.
An acknowledgement (ACK) frame acknowledges receipt of each transmitted data frame. The ACK frame is NAV protected by the preceding data frame, in which the duration field in the data frame specifies a duration value that reserves the medium until the end of the ACK transmission. Alternatively, the first frame transmitted in a signal stream can carry a value in the duration field that covers the entire remaining frames exchanged, possibly comprising multiple data frames and ACK frames. In other words, the duration value covers the subsequent frame exchange, in which each frame exchange is typically one or more pairs of a data frame responded to with an ACK frame.
The virtual carrier sense mechanism, a familiar part of the 802.11 standard, has been previously used to solve a different problem unique to wireless networks. First and second nodes can potentially be separated by a distance greater than their respectively transmitted signals (carriers) can reach, while an intermediate third node can be close enough to each of the first and second nodes to hear both signals.
FIG. 1 depicts telecommunications system 100 of the prior art, comprising nodes 102-1, 102-2, and 102-3. Rings 103-1, 103-2, and 103-3 represent the respective limits of signal coverage for nodes 102-1, 102-2, and 102-3. As depicted, ring 103-1 does not encompass node 102-2, and ring 103-2 does not encompass node 102-1, meaning that the signals from each of the two nodes does not reach the other node. In the example, the intermediate third node (i.e., node 102-3) is already receiving from the first node (i.e., node 102-1), and the second node (i.e., node 102-2) has data packets to transmit. The situation can arise that the second node will not defer its transmission, but instead will also try to transmit and, in the process, potentially corrupt the active transmission from the first node. In the example, nodes 102-1 and 102-2 are essentially hidden from each other.
If a hidden node case is suspected, then 802.11 nodes can invoke an RTS/CTS mechanism of the prior art before any data transmission, depicted in FIG. 2. This means that prior to sending a data frame, a node transmits, as part of its signal stream 201-1, Request to Send (RTS) frame 202, which contains a duration value that covers interval 203 needed for the pending data transmission, including data frame 205 and ACK frame 206. RTS frame 202 will set the NAV locally around the sender using this duration value. If the medium is free around the receiver, it responds, as part of its signal stream 201-2, with Clear to Send (CTS) frame 204, which sets the NAV for all other nodes in the vicinity of the receiver. After the RTS/CTS exchange, other nodes in the areas around the sending and receiving nodes defer their transmission through the virtual carrier sense mechanism.
Although the RTS/CTS mechanism provides interoperability with legacy stations, it is suboptimal because it requires the transmission of two CCK frames (RTS and CTS) prior to the OFDM transmission. The RTS/CTS mechanism is targeted specifically at hidden node situations, in which the area at both the sender and the receiver must be NAV protected, each by a different frame. NAV protection, however, does not necessarily have to be imposed in all OFDM transmissions, especially where it is known that no hidden nodes exist, as shown in the configuration of FIG. 3.
Telecommunications system 300 of the prior art comprises nodes 302-1, 302-2, and 302-3, each with a limit of signal coverage represented by rings 303-1, 303-2, and 303-3, respectively. Note that all three nodes are in each of the three areas of signal coverage, signifying that no hidden nodes exist in the configuration. In such a situation where no hidden nodes exist—a property that can be readily determined—it is disadvantageous to use the additional overhead of the RTS/CTS mechanism.
The need exists for a technique to allow enhanced stations and legacy stations to work with each other without the inefficiencies of signaling overhead in the prior art.